The present invention relates generally to methods for epitaxially depositing a layer of material on a substrate and, more particularly, to an improved method of molecular beam epitaxial (MBE) deposition for reproducibly controlling layer thickness and varying layer composition.
A variety of MBE methods and apparatus have been developed for depositing thin layers of material on substrates. The MBE process is an ultra-high vacuum (e.g. 10.sup.-11 Torr) process whereby group III elements (e.g. Ca, Al, In) and group V elements (e.g. As, P) are heated to form molecular or atomic beams which can than be epitaxially deposited onto a heated substrate as generally described by Tanaka et al in U.S. Pat. No. 4,944,246. Additionally, dopant material (e.g. Be, Cr) can be also deposited.
More recently, the MBE process has been adapted to produce devices having a plurality of controlled layer thicknesses and alternating compositions, e.g. distributed Bragg reflectors (DBR). DBRs are composed of alternating quarter-wavelength (.lambda./4) layers of high and low refractive index materials (e.g. GaAs and AlAs). Generally, controlling layer thickness and composition in the MBE process has involved initially calibrating the MBE system and defining a layer growth rate. Thereafter by simply timing the growth period desired layer thicknesses can be achieved, assuming the growth rate remained substantially constant. However, MBE system operating parameters can vary during a growth cycle as well as from one growth cycle to the next resulting in growth rates which can increase or decrease as a function of time. In particular, changes in source cell temperatures can severely alter the desired growth rate. In fact, when changing source cell temperatures in present MBE systems, the growth rate must be recalibrated after the molecular beam stabilizes, thus a considerable period of time can be lost before the desired growth rate has been reestablished.
An improved MBE method for controlling layer thickness and varying layer composition is described by Igarashi in U.S. Pat. No. 5,096,533 whereby source cell temperature and growth rate can be more accurately controlled. However, layer thickness and composition are controlled by physically interposing a shutter between the source cell and the substrate and opening/closing the shutter to control layer thickness and composition. In such fashion, source cell temperatures can be thus kept at a relatively constant value and thus avoid up and down swings in growth rates resulting from varying source cell temperatures. However, the shuttered MBE source cell operation is still dependent on the assumption of a fixed growth rate so that layer composition and thickness can be controlled simply by controlling the period of deposition.
More recently, Hong et al in, "A simple way to reduce series resistance in p-doped semiconductors and distributed Bragg reflectors", Journal of Crystal. Growth, 111 (1991) 1071-1075 described an improved MBE system which eliminates the need for shutters to control layer thickness and vary composition. Rather than physically shutter the atomic or molecular beam of each source cell, Hong simply varies the source cell operating temperature thus effectively controlling the intensity of the molecular beam between a maximum and a minimum. Much like prior MBE systems though, Hong still depends upon keeping growth rates essentially constant. Quite simply, while Hong teaches using a plurality of MBE growth cycles to achieve a plurality of layers of alternating composition, he still depends upon maintaining a constant growth rate during the manufacture of a single DBR.
In spite of such advances, MBE processes persist to operate under the assumption that constant growth rate will yield controlled layer thicknesses and compositions. The essence of such assumption is that variations in growth rate during the MBE process cannot be tolerated. Consequently, a need remains for an MBE process which can reliably reproduce specified layer thicknesses and for controlling layer compositions without depending on the assumption of constant growth rate and the resulting need to recalibrate growth rates periodically. Moreover, a need exists to reliably produce a plurality of devices with the MBE process. This need has most recently arisen in the field of DBRs for vertical-cavity surface-emitting lasers (VCSELs) which can require center reflection wavelengths with a tolerance of at least .+-.1-2%. Additionally, a need has arisen for an MBE process to produce DBRs having reduced heterojunction resistance.